Scenarios, in most situations, are descriptions of required interactions between a desired system and its environment, which detail normative system behaviour. Our studies of current scenario use in requirements engineering have revealed that there is considerable interest in the use of scenarios for acquisition, elaboration and validation of system requirements. However, scenarios have seldom been used to study inappropriate or exceptional system behaviour. To account for non-normative or undesired system behaviour, it is vital to predict (‘what can go wrong’) and explore the existence or occurrence of ‘exceptions’ in a scenario when the system System, here, is not just a pure software system but is a socio-technical system which has social technical and organisation dimensions with the user as an integral part of the system. In this article, we present a prototype software tool called CREWS–SAVRE for systematic scenario generation and use. We describe the innovative features of the tool and demonstrate them with an example of tool’s use. Further, we have identified three kinds of exceptions: generic, permutation and problem exceptions, and have derived complex taxonomies of problem exceptions. We have populated SAVRE with the taxonomies of generic, permutation and problem exceptions. The exceptions can be chosen by the requirements engineer to include them in the generated scenarios to explore the correctness and completeness of requirements. In addition, the taxonomies of problem exceptions can also serve as checklists and help a requirements engineer to predict non-normative system behaviour in a scenario.

Scenarios have been advocated as a means of improving requirements engineering yet few methods or tools exist to support scenario based RE. The paper reports a method and software assistant tool for scenario based RE that integrates with use case approaches to object oriented development. The method and operation of the tool are illustrated with a financial system case study. Scenarios are used to represent paths of possible behavior through a use case, and these are investigated to elaborate requirements. The method commences by acquisition and modeling of a use case. The use case is then compared with a library of abstract models that represent different application classes. Each model is associated with a set of generic requirements for its class, hence, by identifying the class(es) to which the use case belongs, generic requirements can be reused. Scenario paths are automatically generated from use cases, then exception types are applied to normal event sequences to suggest possible abnormal events resulting from human error. Generic requirements are also attached to exceptions to suggest possible ways of dealing with human error and other types of system failure. Scenarios are validated by rule based frames which detect problematic event patterns. The tool suggests appropriate generic requirements to deal with the problems encountered. The paper concludes with a review of related work and a discussion of the prospects for scenario based RE methods and tools.